Microfibrillated cellulose (MFC), which also is known as nanocellulose, is a material typically made from wood cellulose fibers. It can also be made from microbial sources, agricultural fibers, dissolved cellulose or CMC etc. In microfibrillated cellulose the individual microfibrils have been partly or totally detached from each other.
Microfibrillated cellulose has a very high water binding capacity and it is thus very difficult to reduce the water content of a slurry comprising microfibrillated cellulose and accordingly it is thus difficult to purify. High water content of a slurry comprising microfibrillated cellulose also prevents usage of MFC in many different application where MFC with high solids would be required.
Today there exist several different methods to remove water from a slurry comprising cellulose, such as microfibrillated cellulose. It is for example possible to use different drying techniques. Examples of different drying techniques are; freeze drying, spray drying and supercritical drying. These techniques are however quite energy demanding and thus not so cost efficient to use in large scale processes. Also, hornification, or superhornification, of the microfibrillated cellulose fibers often tends to occur when water is removed with different drying techniques. Hornification is when irreversible bonds between the fibers are formed. When hornification has occurred it is not possible for the fibers to expand and swell in water and the original water bonding capacity of the fibers is thus lost. The hornification may be prevented by addition of chemicals which physically prevent or modify the fibers in such way that the formation of bonds between cellulose fibers are limited or prevented. CA1208631A describes a process to re-disperse dried microfibrillated cellulose by addition of additives that will prevent the fibrils from bonding to each other and thus also prevents hornification of the fibers.
Further there is disclosed by Luchache et al. in Annals of the University of Craiova, Electric Engineering series, No. 32, 2008; ISSN 1842-4805 dewatering of pulp and paper waste sludge.
Mechanical treatments in order to remove water from a slurry comprising cellulose, such as microfibrillated cellulose can also be used. However, they are normally not very successful due to the small fiber size and size distribution of the microfibrillated cellulose. Moreover, filtration of a slurry comprising cellulose, such as microfibrillated cellulose is difficult due to the dense web formed by the slurry. Furthermore, the bonds between the microfibrillated cellulose fibers are also quite strong and this will also make mechanical dewatering less efficient.
The inefficiency or limitations in drying in e.g. pressurized dewatering will further give problems with the removal of ions of cellulose constituents. Since a filter cake is formed during dewatering, a higher resistance to dewatering is obtained. At the same time, it is more difficult to remove e.g. ions or other dissolved species since these might be accumulated in the filter cake. Therefore, the obtained dewatered filter cake of MFC might in fact contain the initial amount of ions or even substantial higher amount of ions.
When using a normal drying method, the ions and residual chemicals will remain in the concentrated fiber suspensions and finally in the dried MFC or cellulose sample.
There is thus a need for an improved process for purifying, such as salt/ion depletion and/or free sugar depletion, of a slurry comprising cellulose, such as microfibrillated cellulose without causing hornification, or superhornification.